In the air quality directive (2008/EC/50) the EU has set two limit values for nitrogen dioxide (NO2) for the protection of human health: the NO2 hourly mean value may not exceed 200 micrograms per cubic metre (µg/m3) more than 18 times in a year and the NO2 annual mean value may not exceed 40 micrograms per cubic metre (µg/m3). These limit values come into force for concentrations measured from 1.1.2010 so during 2009 a margin of tolerance equal to an annual mean value of 42 micrograms per cubic metre (μg/m3) is still in place.